Give me an example of how a biotic factor minght influence the organisms in an ecosystem

A biotic factor such as a disease will:
- kill off most of a certain species (if is a prey) -> less prey/food for predators
- predators starve/die -> less predators to control other quickly reproducing prey species -> however less competitors for other predators as well
e.g. rabbits and hawks 

A disease which kills lots of hawks (predators of rabbits i think) means a rabbit plague can incur since there is no other natural (i think) predators which can keep their population at appropriate levels (through hunting them as food)
(Thus note that biotic factors can have positive or negative consequences)